6.0 Summary and Plans for Further Development

     A contaminant source-term release module has been developed for MEPAS. This report documents the implementation of this computer module for MEPAS Version 3.2 and RAAS Version 1.1. Contaminated aquifer, pond/surface impoundment, and vadose zone source zones can be analyzed. Contaminants can be present in aqueous, sorbed, vapor, or NAPL phases. The module calculates the simultaneous contaminant fluxes (as functions of time) to various loss routes (i.e., degradation/decay, leaching, wind suspension, water erosion, overland flow, and/or volatilization) in a manner that accounts for interactions between loss processes and conserves overall mass. The outputs of the source-term release module are linked as inputs to the environmental transport, and exposure/health impact modules of MEPAS.

     The source-term release module can be used to simulate "baseline" scenarios as well as scenarios where certain remediation methodologies have been implemented at the site. Remediation methodologies that can be simulated include all methodologies that can be represented merely by changing the magnitude of certain model parameters, as well as ISV and ISS methodologies in the vadose zone (which use different theory implemented in the module).

     Because a numerical solution scheme is used to solve the differential equations of contaminant loss versus time, certain environmental input parameters can be entered as time-varying quantities rather than being constrained to constants. Analytical solutions that apply to special cases are also presented in the report.

     For certain simpler scenarios, the numerical solution algorithms implemented in the module have been verified through comparison with analytical solutions. Tests of the algorithms for calculating loss to different routes have been conducted through applications of the source-term module to specific problem scenarios. Additional testing is in progress.

     The current version of the source-term release module represents a consolidation and significant augmentation of numerous source-term release capabilities that existed in different modules of previous versions of MEPAS. However, it does not represent the ultimate source-term release capability envisioned for this component. The current version contains a number of idealizing assumptions and limitations. The remainder of this section discusses these, and describes further development plans envisioned for the module.

     The current version of the module associated with RAAS Version 1.1 limits the contaminants that can be analyzed to those that already exist in the associated RAAS contaminant property database. In the future, the module may be modified to allow the user to analyze contaminants not already in the database (without actually modifying the database). In this event, values of all relevant contaminant-specific properties would have to be supplied by the user. (The capability to edit the MEPAS database already exists.)

     The current version of the module determines whether a contaminant is one that may partition into a NAPL phase by using the database value of the modified (unitless) Henry's Law constant as a test metric (i.e., NAPL phase contaminant if K_Hi > 10^-7). This method makes appropriate determinations in most instances; but does lead to errors in cases that include nonvolatile organic compounds (e.g., PCBs) or volatile radionuclides (e.g., tritium and iodine). In the future, the module may be modified to include a new method to make this determination, either using the database value of the octanol-water partition coefficient as a test metric or (preferably) using the chemical's category number from the chemical classification system currently built into the RAAS software.

     The current version of the module uses a relatively complex, yet still simplified, method of determining aqueous and vapor concentrations for a contaminant (at each time step) in systems where a NAPL phase exists. In the future, the module may be modified to include a full phase distribution model for this calculation (at each time step), which means that all volumetric fluid contents (water, air, NAPL) will be recalculated simultaneously with the contaminant phase partitioning.

     This report presents analytical solutions applicable to special cases; however, they are not currently implemented in the module. In the future, these analytical solutions may be implemented so that the user can directly verify or bypass numerical outputs of the module.

     Idealized theory for volatilization from a pond/surface impoundment source zone has been implemented in the module to act as a bounding calculation for highly volatile compounds that tended to disappear too quickly (with the 1-yr time step commonly used in simulations). However, simulated volatilization rates for these compounds still seem high. In the future, the module may be modified to correct this situation by either adding a capability to vary the length of the time step automatically during a simulation based on limits on the amount of mass that can be lost during a time step, or by implementing improved models for volatilization for the pond/surface impoundment source zone.

     Finally, previous versions of MEPAS included the capability to analyze additional types of volatilization scenarios beyond what are currently implemented in the module. In the future, the module may be augmented to address these scenarios as well. These scenarios are 1) a landfill without internal gas generation, 2) a landfill with internal gas generation, 3) an aqueous pond/surface impoundment that contains a layer of contaminated sediment at the bottom (which controls the volatilization loss), and 4) a surface spill of liquid that is a NAPL-phase mixture.